If you look at Zip's latest quarterly, US app downloads were...

If you look at Zip's latest quarterly, US app downloads were 1.5m, and new customers were 600k, which also includes new customers from app downloads and those generated at merchant checkouts. Even if all 600k were from app downloads that's a conversion rate of only 40%, which as said is probably incorrectly high, with the real figure maybe being closer to 30% or so (to account for new sign-ups at checkouts).
By comparison IOU activation rate was 28.3% of download totals, which is pretty similar to Zip US, noting again IOU would have had some new customers coming from integrated checkouts. I think you'll find many of the BNPLs have a similar conversion rate as customers download the app, search, browse etc but won't make a purchase straight away.